The 91-year-old great-great-great-granddaughter of shipping magnate Cornelius Vanderbilt was dubbed America’s “poor little rich girl” and maybe is best known as the subject of a sensational custody battle, the woman who witnessed one of her sons commit suicide – and those jeans. And yet, Cooper said, while interviewing his mother for the docu Nothing Left Unsaid, he was struck by the fact that that she is “far more joyous than me.”

“She’s like Henry VIII and I’m like Cromwell. She’s the most youthful person I know, even at almost 92,” Cooper said of his mother, who was on stage with filmmaker Liz Garbus. He appeared via satellite before hosting a CNN town hall meeting on guns with President Obama.

The film opens with footage Cooper shot of his mom on a beach in 1990, two years after his brother Carter took his own life. Garbus discovered the footage among many boxes of material Cooper unearthed in a storage unit Vanderbilt has had for his entire life. “I don’t know if you are familiar with Citizen Kane,” Cooper said, reminding TV critics of the famous storage room scene in the Orson Welles classic. Going through his mother’s storage unit, one box would contain “amazing letters from Howard Hughes, who dated her when he was Hot Howard Hughes before he was Desert Inn Howard Hughes.” In another box, he found a box of cornflakes she’d forgotten to throw away when packing to move.

“It sounds like the show Hoarders,” Vanderbilt complained. “It was not like that!”

“It was like that,” Cooper mouthed to TV critics. (Later, however, realizing he was “in trouble” with his mom, he insisted: “It’s really not like Hoarders. It’s a really nice room.”)

The most interesting take-away from the documentary, Cooper said, is that his mother “comes from time and place that doesn’t exist any longer. She was born into this family that on paper was one of the richest in America, living in palaces … enormous mansions they called ‘cottages.’ That kind of world doesn’t exist, and many of the people my mom has encountered along the way – they’re no longer alive in many cases, and their stories are no longer remembered.”

The docu, Cooper promised, “shines a light on stories not being told” anymore. He recalled watching an Errol Flynn movie with his mother and asking her, “Did you ever know Errol Flynn?” To which she responded, “Oh, yes.”

“But I always knew it was a lot more than that,” Cooper said, noting that she dated Flynn when she was 17.”I did date him, once,” Vanderbilt said, as her son made air quotes, and HBO’s docu chief Sheila Nevins jumped in to cut off the Q&A, to the dismay of the critics. As Nevins wrapped things up, Cooper could be heard saying to his mother, “Tell them about your ‘date’ with Marlon Brando.”